Medical researchers in the 21st century will not be content simply to treat illness and disease - they will aim to cheat death itself.

Scientists are now close to understanding the biological mechanisms that make us age and die. Their work is examined in a special edition of the BBC programme Horizon broadcast on Tuesday.

A few decades ago, no-one thought we could add years to life.

I realised for the very first time we were able to put cellular ageing on hold

Dr Jerry Shay, University of Texas

The maximum lifespan possible for humans was believed to be a hundred or so - and all because of an immutable genetic clock.

But recently a series of discoveries has forced scientists to rethink their theories on ageing.

They have already found some of the genes involved in delaying the ageing process - in lab animals.

Fruitflies have defied death in the lab

Tinkering with them, geneticists have created races of super-organisms - for instance, fruit flies that can live double their natural lifespan and that die healthy and vigorous.

In other labs they have discovered mice that can spontaneously regenerate parts of their bodies, constantly repairing the damage that is part of the ageing process.

Controversially, some scientists are confident that their results can be extended to future generations of humans.

Understanding ageing

Progeric children age prematurely

Through studying the rare condition of Progeria, which seems to dramatically accelerate the ageing process in children and causes them to age and die tragically young, scientists have gained further insights into what causes our cells to stop dividing and start to die.

They have discovered ageing is controlled by a piece of DNA called a telemere that stops the DNA from fraying every time it divides.

After every cell division, however, the telomere gets shorter. Eventually, it shortens to a critical length, and can no longer protect the DNA from fraying. At this point the cell dies.

Progeric children have unnaturally short telomeres, and so cell death is accelerated, thus speeding up the ageing process.

Some scientists have been able to briefly reverse the process - in cells in a test tube.

They did this by genetically engineering an enzyme called telomerase which can re-construct telomeres. However, it is not found naturally in most adult cells, only exists in sperm and egg cells and the developing foetus.

Biologist Dr Jerry Shay, of the University of Texas, worked on the study. He said: "I realised for the very first time we were able to put cellular ageing on hold.

"If you introduce telemorase into normal cells it keeps the cells going and going and going"

But will this ever become a feasible treatment in the future?

Noone knows how to insert genes into every cell of a living person. It is was possible, there would also be a risk of cancer, which is caused by cells that fail to die and continue to divide in an uncontrolled fashion.

Currently, the most promising advance comes from researchers who have discovered how to inject human stem cells - unique foetal cells that have the power to repair or replace any tissue in the human body - into the brain of stroke victims, partially reversing their brain damage.

In the future, this process might reverse many aspects of the ageing process.

There is already a selection of hormones, diets and therapies on offer, all of which claim to extend life.

Some of them help. Most of them don't. Through health care, and modern medicine, we have already made ourselves live longer than evolution dictates.

The question is will any of the more extreme and futuristic areas of research give us immortality in the 21st century?

If we did not die then we would be around competing with our children and that is not very good for evolution

Professor Lee Silver, Princeton University

Professor Lee Silver, a geneticist at Princeton University, says any attempt to achieve immortality would go against nature.

He said: "Death makes perfect sense in terms of evolution, in terms of passing your genes on to the next generation.

"If we did not die then we would be around competing with our children and that is not very good for evolution."

Horizon, Life and Death in the 21st Century, is broadcast on BBC2 at 20 00 GMT on Tuesday 4 January.